The invention relates generally to the reproduction of a master.
More particularly, the invention relates to a method of and an apparatus for masking a master, especially a photographic master, preparatory to reproduction.
When photographs have large brightness variations in individual regions, the bright zones are frequently overexposed and the dark zones frequently underexposed in copies of the photographs. Consequently, details and fine structures are very poorly, or not at all, reproduced on the copies.
The German Offenlegungsschrift 31 41 263 describes a method of copying color diapositives on reversal paper using masks for contrast reduction. The diapositive is placed in direct contact with phototropic glass and ultraviolet light or the like is then passed through the diapositive into the glass. A black-and-white mask representing a negative of the diapositive is thus produced in the phototropic glass. The composite of mask and diapositive is thereupon illuminated in the opposite direction while remaining in the same position. The diapositive is thereby reproduced on the reversal paper with low contrast.
Due to the direct contact between the masking glass and the diapositive, a relatively sharp mask is produced in this method and is sharply imaged on the reversal paper. To obtain copies of high quality, it is necessary for the mask and the diapositive to be practically one hundred percent in register during exposure. However, this is precluded because the materials, namely, the film material and the glass, are different. During the very intensive exposure, the materials are heated relatively strongly and expand to different degrees. The copies obtained are then of lower quality and the dark/bright gradations of the mask which do not precisely register with the dark/bright gradations of the diapositive are clearly visible.
If, as described in the Offenlegungsschrift, the method is employed in a large laboratory with automatic copiers, additional difficulties arise because of rapid, jerking movements to which the composite of masking glass and diapositive is subjected between the individual stations. Shifting can again occur and even more strongly influence the quality of the copies.